prom_free_prom_memory() returns unsigned long

Some boards declare prom_free_prom_memory as a void function but the
    caller free_initmem() expects a return value.
    
    Fix those up and return 0 instead, just like everyone else does.
    
    Signed-off-by: Arthur Othieno <a.othieno@bluewin.ch>
    Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/momentum/jaguar_atx/prom.c

@@ -236,8 +236,9 @@ void __init prom_init(void)
 #endif
 }
 
-void __init prom_free_prom_memory(void)
+unsigned long __init prom_free_prom_memory(void)
 {
+	return 0;
 }
 
 void __init prom_fixup_mem_map(unsigned long start, unsigned long end)

arch/mips/momentum/ocelot_3/prom.c

@@ -239,8 +239,9 @@ void __init prom_init(void)
 #endif
 }
 
-void __init prom_free_prom_memory(void)
+unsigned long __init prom_free_prom_memory(void)
 {
+	return 0;
 }
 
 void __init prom_fixup_mem_map(unsigned long start, unsigned long end)

arch/mips/pmc-sierra/yosemite/prom.c

@@ -132,8 +132,9 @@ void __init prom_init(void)
 	prom_grab_secondary();
 }
 
-void __init prom_free_prom_memory(void)
+unsigned long __init prom_free_prom_memory(void)
 {
+	return 0;
 }
 
 void __init prom_fixup_mem_map(unsigned long start, unsigned long end)